1. Field of the Invention
The present invention relates generally to the fabrication of ophthalmic lenses to be mounted in a spectacle frame.
It relates more particularly to a process for fabricating a surface of an ophthalmic lens, in accordance with a given prescription, an installation for implementing the process and an ophthalmic lens having at least one surface obtained by the process.
The surface to be formed can be spherical, aspherical, toric, atoric or more generally a complex surface such as a progressive surface, although the invention is not limited to this type of surface.
2. Description of the Prior Art
Highly sophisticated machines are available for fabricating a surface of an ophthalmic lens having an acceptable optical quality in a single machining step. This implies a very slow rate of advance of the machining tool, a very small overlapping pitch of the tool over the surface to be machined, very precise positioning of the machining tool on said surface and a very long machining time.
These high-precision machines are relatively costly and the machining times entailed are long, which leads to a cost of production of ophthalmic lenses that would be prohibitive on an industrial scale.
The present invention therefore proposes a new process for quickly forming a surface of an ophthalmic lens in accordance with a given prescription using less accurate machines but obtaining a lens surface of good optical quality and which also satisfies cosmetic requirements.
To be more precise, the process of the invention includes the following steps:
a) machining the surface by removing material by moving a cutting tool along a continuous trajectory in a surface envelope to be achieved to form grooves, two adjacent grooves being spaced by a constant pitch from 0.01 mm to 3 mm, to obtain a surface having an arithmetic mean roughness Ra from approximately 0.1 xcexcm to approximately 0.7 xcexcm, and
b) smoothing the machined surface by moving a smoothing tool along a continuous trajectory formed of passes within a surface envelope to be achieved, two adjacent passes being spaced by a constant pitch from 0.2 mm to 3 mm, to effect band-pass filtering of the undulations of the surface between a low frequency corresponding to the surface envelope and a high frequency corresponding to a background roughness, to obtain a smoothed surface having an arithmetic mean roughness Ra less than 0.1 xcexcm.
In the process of the invention, a smoothed surface having a roughness Ra from 0.05 xcexcm to 0.07 xcexcm is preferably obtained in step b).
The trajectory of the cutting tool and the trajectory of the smoothing tool in steps a) and b) are advantageously spirals.
The process according to the invention can use a semifinished lens having one molded face, for example, the other face being adapted to the prescription of the wearer.
It is equally possible to use a semifinished lens of which both spherical faces are parallel, in which case both surfaces are modified by the process of the invention.
After step b) the process of the invention advantageously includes a step c) of applying to the smoothed surface a layer of varnish to confer a polished surface state on the surface.
This latter step c) replaces the polishing operation which is slow and tiresome and which necessitates a special tool for each surface geometry.
The thickness of the layer of varnish deposited on the surface in step c) can be from approximately 500 Ra to approximately 800 Ra, where Ra corresponds to the surface roughness obtained in step b).
The minimum thickness of the layer of varnish deposited on the surface in accordance with the invention is from approximately 30 Ra to approximately 200 Ra, where Ra corresponds to the roughness of the surface obtained in step b).
The varnish deposited in the liquid state on the surface by the process of the invention preferably has a viscosity at 25xc2x0 C. from 1000 mPas to 3000 mPas.
According to the invention, the machining process of step a) can be milling or turning.
In the case of turning, a vertical lathe such as the SCHNEIDER HSC 100 CNC machine can be used, for example. A machine of this kind is described in patent application EP 0 849 038.
The invention also concerns an installation for implementing the aforementioned process, the installation including a numerically controlled machining machine tool including a lens support mobile in translation along a vertical axis and adapted to be driven in rotation about a vertical axis and a milling tool mobile in translation along a horizontal axis, inclinable relative to a vertical axis and driven in rotation about a vertical axis, the advance motions of the support and the tool, i.e. the translation and rotation motions of the support and the translation and inclination motions of the tool, being controlled simultaneously by the numerical control unit, the speed and rotation of the milling tool being controlled by the numerical control unit.
The installation advantageously further includes a numerically controlled smoothing machine tool including a lens support mobile in translation along a vertical axis and adapted to be driven in rotation about a vertical axis and a smoothing tool mobile in translation along a horizontal axis, inclinable relative to a vertical axis and driven in rotation about a vertical axis, the advance motions of the support and the tool, i.e. the translation and rotation motions of the support and the translation and inclination motions of the tool, being controlled simultaneously by the numerical control unit, the speed of rotation of the smoothing tool being controlled by the numerical control unit.
The smoothing tool of the smoothing machine in accordance with the invention preferably includes a rigid support and a tool fastened to the support which is annular in shape and whose dimensions are small compared to those of the surface to be smoothed but relatively large relative to the surface defects to be eliminated.
The tool includes the following superposed components: an elastically deformable core having a Shore A hardness from 30 to 80xc2x0, an elastically deformable surface layer whose elasticity is less than that of the core, and an abrasive film or a support driving an abrasive which forms the working surface of the tool.
The installation in accordance with the present invention advantageously further includes a machine for applying a layer of varnish to the smoothed surface of the lens, which machine includes a lens support adapted to be driven in rotation and an arm carrying a nozzle for distributing the liquid varnish at a low pressure adapted to be moved in translation vertically and horizontally relative to the support.
The following description, which is given by way of non-limiting example and with reference to the accompanying drawings, explains in what the invention consists and how it can be put into effect.